Ta có:
$A= \dfrac{1}{4} + \dfrac{1}{28} + \dfrac{1}{70} + \dfrac{1}{130} + \dfrac{1}{208} + \dfrac{1}{304}$
$⇔3A = \dfrac{3}{1.4} + \dfrac{3}{4.7} + \dfrac{3}{7.10} + \dfrac{3}{10.13} + \dfrac{3}{13.16} + \dfrac{3}{16.19}$
$⇔3A= \dfrac{1}{1} - \dfrac{1}{4} + \dfrac{1}{4} - \dfrac{1}{7} + \dfrac{1}{7} - \dfrac{1}{10} + \dfrac{1}{10} - \dfrac{1}{13} + \dfrac{1}{13} - \dfrac{1}{16} + \dfrac{1}{16} - \dfrac{1}{19}$
$⇔ 3A = 1 - \dfrac{1}{19}$
$⇔ 3A = \dfrac{18}{19}$
$⇔ A = \dfrac{6}{19}$
